Thorium vapor pressure

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Proceeding from thorium to plutonium along the actinide series, the vapor pressure of the corresponding iodides decreases and the thermal stability of the iodides increases. The melting point of U metal is below 1475 K and for Np and Pu metals it is below 975 K. The thermal stabilities of the iodides of U, Np, and Pu below the melting points of the respective metals are too great to permit the preparation of these metals by the van Arkel-De Boer process. 

The phases of thorium metal and their transition temperatures are listed in Table 6.4. Equations for the vapor pressure of thorium metal are [11 ]... 

Alternative processes for preparing metallic americium are the reduction of AmFs with barium vapor in high vacuum at about 1300°C, reduction of AmF4 with calcium, and reduction of Am02 with lanthanum or thorium at about ISOO C in high vacuum. The vapor pressure of americium is much higher than that of lanthanum or thorium, so that pure americium is condensed in the colder parts of the apparatus [K2, L2]. Metallic americium dissolves readily in mineral acids. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

Thorium acetylacetonate is insoluble in water and soluble in many organic solvents. It is converted by acids into acetylacetone and the thorium salt of the acid used. Its vapor pressure at 100° is 3.2 + 0.3 X 10 mm. 

While improvements in results were exhibited when applying the above generalizations, Meissner and Kusik (M7) suggested, in 1978, using the method for zinc and cadmium chlorides, bromides and iodides, sulfuric acid, and thorium nitrate only when experimental data is unavailable. The user should also be aware that estimating T from vapor pressures and extrapolation over a wide ionic strength range can increase the calculation error. 

The old method of heating the calcium salts of formic and a second carboxylic acid for aldehyde formation has been modified by the use of a catalytic decomposition technique. By this scheme, the acid vapors are passed over thorium oxide, titanium oxide, or magnesium oxide at 300° or the acids are heated under pressure at 260° in the presence of titanium dioxide. In the latter procedure, non-volatile acids can be used. With aliphatic acids over titanium oxide, reaction occurs only when more than seven carbon atoms are present, the yields increasing with increase in the molecular weight (78-90%). Aromatic-acids having halo and phenolic groups are converted in high yields to aldehydes, e.g., salicylaldehyde (92%) and p-chlorobenzaldehyde (8S>%). Preparation of a thorium oxide catalyst has been described (cf. method 186). 

While the cooled flask is rotating (pressure <10-6 torr) the 1 % thorium-doped tungsten filament is brought carefully (outgassing) to 16-18 A. A 2-kV hearth voltage giving rise to 30-mA emission current is sufficient to vaporize 0.3 g of vanadium from a 0.7-g bead over a 30-min period ( 0.2 mmole min-1). After the vaporization has been terminated, the product is isolated as described above. 

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